Research Dossier on IGF1-LR3

(Growth Factor Peptide)

Classification & Molecular Identity

Amino acid sequence, molecular weight, structural motifs

IGF-1 LR3 (Long Arg³ IGF-I) is a recombinant analog of human IGF-1 engineered to reduce high-affinity sequestration by insulin-like growth factor-binding proteins (IGFBPs) and thereby enhance bioactivity in experimental systems. LR3 retains the 70-residue core of human IGF-1 but incorporates two key modifications relative to mature IGF-1:

1. a single amino-acid substitution at position 3 (Glu³→Arg³, hence “R³”), and

2. an N-terminal extension of 13 amino acids (“Long”) derived from the pre-propeptide region. Together these changes alter surface charge and local conformation around IGF-1 residues involved in IGFBP recognition while preserving high-affinity binding to IGF-1 receptor (IGF1R). Multiple biochemical descriptions list LR3 as an 83-amino-acid polypeptide (70 native residues + 13-residue extension); the precise molecular mass depends on expression construct and processing. GenScript+1

Coordination and fold. NMR/biophysical analyses indicate that Long-[Arg³] IGF-I adopts an IGF-1–like fold (insulin/IGF family), with B-, C-, A- and D-domains forming the characteristic compact structure required for IGF1R binding. Structural work suggests that reduced IGFBP binding reflects not only the charge reversal at position 3 but also subtle spatial rearrangements of surface loops that contribute to IGFBP contacts. ScienceDirect

Discovery history (lab, year, species)

IGF-1 was cloned and its receptor characterized through the 1980s–1990s; structure–function studies then mapped residues responsible for receptor vs IGFBP binding, inspiring analogs with attenuated IGFBP affinity. Substitutions in the A- and B-domains and targeted charge changes (including Arg³) were evaluated in vitro and in vivo, leading to the Long Arg³ variant that became widely used as a cell-culture supplement and as a tool compound in animal endocrinology. PubMed

Endogenous vs synthetic origin

IGF-1 is endogenous, produced primarily by the liver and locally in many tissues under growth hormone (GH) control. IGF-1 LR3 is fully synthetic/recombinant, not a naturally occurring human sequence, and is engineered specifically for altered IGFBP interactions while retaining IGF1R agonism. PubMed

Homologs, analogs, derivatives

• Native IGF-1 (70 aa) and IGF-2 (67 aa), both bind IGF1R and IGF2R (the latter a clearance receptor).

• Other IGF-1 analogs (e.g., Arg³ IGF-1 without the N-terminal extension; domain-swap or point-mutant libraries) helped define IGF1R vs insulin receptor specificity. PubMed

• Mecasermin (recombinant human IGF-1, rhIGF-1) is a clinically developed native-sequence agonist (distinct from LR3). No licensed medicinal product uses the LR3 sequence. (Not established for clinical development.)

• Cell-culture grade LR3 is commercially manufactured as a serum-replacement growth factor to drive proliferation/survival under low-serum conditions. (Vendor statements are not clinical evidence; used only to contextualize research usage.) Repligen

Historical Development & Research Trajectory

Key milestones in discovery and study

• Binding-protein biology (1990s). IGFBPs 1–6 were recognized as high-affinity extracellular carriers that buffer and localize IGFs, often with IGF1R-level affinity, thus limiting free ligand; this spurred design of variants with lower IGFBP affinity. PubMed+1

• Arg³/Long Arg³ analogs. Early animal studies reported potent trophic effects of Arg³/Long-Arg³ IGF-1, but outcomes were model-specific; in pigs, continuous infusion or repeated dosing paradoxically reduced growth rate and endogenous IGF-axis readouts despite organ growth signals, highlighting species- and context-dependent physiology. (Investigational doses used in studies Conlon 1995; Dunaiski 1998.) PubMed+2PubMed+2

• Cell-culture adoption. LR3 became a standard serum-free supplement in biotech manufacturing (CHO/HEK lines) because reduced IGFBP binding avoids sequestration by IGFBPs secreted into the medium, typically improving viable cell density and recombinant protein yield. (Again, this is in vitro process data, not clinical.) Repligen

Paradigm shifts and controversies

1. IGFBPs regulate more than “availability.” The later literature reframed IGFBPs as dynamic modulators with cell- and matrix-dependent roles (sometimes enhancing IGF action by presenting ligand to receptors), complicating the early narrative that “less IGFBP binding is always better.” PMC

2. Species/axis feedback. Reducing IGFBP binding alters feedback loops (GH–IGF–IGFBP-3/ALS ternary complex). In pigs, Long R³ IGF-I lowered plasma IGF-I, IGF-II, and IGFBP-3 and inhibited net growth even as some organs enlarged—an instructive reminder that endocrine set-points matter. (Investigational doses; see below.) PubMed

3. Clinical vacuum. Unlike native IGF-1 (mecasermin), LR3 has not proceeded through modern clinical development; registered interventional trials specifically of LR3 are not found as of September 26, 2025. (Not established for human PK/PD, efficacy, or safety.)

Evolution of scientific interest

Current interest centers on IGF1R signaling mechanics, crosstalk with the insulin receptor (IR/IGF1R hybrids), and IGFBP biology; LR3 remains primarily a research tool for in vitro and selected animal studies, and as a bioprocess supplement where IGFBPs would otherwise deplete free IGF-1. Nature+1

Mechanisms of Action

Primary and secondary receptor interactions

Primary target: the insulin-like growth factor-1 receptor (IGF1R), a heterotetrameric receptor tyrosine kinase. Ligand binding induces autophosphorylation and activation of downstream pathways (RAS–MAPK and PI3K–AKT–mTOR), promoting survival, growth, and differentiation programs. LR3’s R³ substitution and N-extension do not abolish IGF1R affinity; in most reports LR3 retains high-affinity agonism at IGF1R while showing reduced binding to IGFBPs. PMC+1

IGFBP interactions: Native IGF-1 circulates largely bound to IGFBP-3/acid-labile subunit (ALS) as a ternary complex with long residence time; IGFBPs can both inhibit and facilitate IGF action, depending on matrix binding and proteolysis. LR3 was designed to diminish high-affinity IGFBP binding, increasing the free fraction (especially in media or interstitial fluid containing IGFBPs), though exact dissociation constants vs each IGFBP vary by method. PMC+1

Intracellular signaling pathways

Upon IGF1R activation, LR3 engages canonical cascades:

• PI3K–AKT–mTOR → protein synthesis, survival (FOXO inhibition), metabolism;

• RAS–RAF–MEK–ERK → proliferation/mitogenesis;

• Additional wiring includes SHP2/GRB2, IRS1/2 scaffolding, and crosstalk with integrins and GPCRs, with context-dependent nuclear trafficking of IGF1R discussed in cancer signaling. PMC+1

CNS vs peripheral effects

Peripheral effects dominate: skeletal muscle, liver, adipose, connective tissue. CNS penetrance of large polypeptides like IGF-1 (and LR3) across the intact BBB is limited; in vivo CNS effects typically rely on local production, BBB transport mechanisms, or disruption (disease models). Direct brain exposure of LR3 in humans: Not established.(General IGF1R biology in the CNS is documented, but LR3-specific CNS PK remains unknown.) PMC

Hormonal, metabolic, immune interactions

LR3—like IGF-1—interfaces with the GH–IGF axis (feedback to GH secretion), glucose–lipid metabolism, and immune cell growth/survival pathways. Animal studies with Long R³ IGF-I demonstrate axis feedback (changes in endogenous IGF-I/II, IGFBP-3, GH) and organ-specific trophic effects; however, direction of whole-body growth can diverge by species and dose schedule. PubMed+1

Evidence grading (A–C)

• A (replicated core biology): IGF1R signaling via PI3K–AKT and MAPK; IGFBP regulation of IGF bioavailability; LR3’s reduced IGFBP affinity relative to native IGF-1. PMC+2PMC+2

• B (model-specific outcomes): LR3 effects on growth/organ size/endocrine feedback in animal models; enhanced in-vitro potency as a cell-culture supplement. PubMed+1

• C (hypothesis/unknown): Human pharmacokinetics, clinical efficacy, and long-term safety for LR3; CNS delivery under physiological BBB conditions. (Not established.)

Pharmacokinetics & Stability

ADME profile

• Absorption & distribution: For native IGF-1, most circulating ligand is sequestered in IGFBP-3/ALScomplexes, extending residence time. By reducing IGFBP affinity, LR3 is expected to increase free/interstitial fractions; whether this yields longer or shorter systemic half-life in vivo is not established in humans (trade-off between less sequestration vs less ternary-complex stabilization). Quantitative human PK of LR3: Unknown.PubMed

• Metabolism & excretion: Large polypeptides are cleared by receptor-mediated endocytosis and proteolysis; renal filtration of intact LR3 is unlikely due to size, but fragments may be excreted renally. In vivo metabolite maps for LR3: Not established.

• Comparators: For context, IV native IGF-1 and complexed IGF-1 display markedly different half-lives; the ternary complex can persist hours, whereas free IGF-1 is rapidly cleared. The net effect for LR3 depends on competing processes (less binding to IGFBPs vs receptor-mediated clearance). (Human data lacking for LR3.) Europe PMC

Plasma half-life & degradation pathways

No peer-reviewed human half-life has been definitively published for LR3 under standardized conditions. In vitro/cell-culture stability is reported by vendors to exceed insulin under standard serum-free conditions, but those claims (e.g., “two-fold more stable than insulin in culture”) do not substitute for in-vivo PK. (Not established in humans.) Repligen

Stability in vitro & in vivo

• In vitro: LR3 is widely used in serum-free culture specifically because reduced IGFBP binding prevents sequestration by IGFBPs shed into the medium, effectively prolonging functional availability to cells. Repligen

• In vivo: Proteolysis, receptor-mediated internalization, and endocrine feedback are expected to dominate. Validated in-vivo stability data for LR3 in humans: Not established.

Storage/reconstitution considerations

Peer-reviewed publications do not provide standardized, product-agnostic reconstitution/stability curves for LR3 research vials; general peptide/protein handling principles (cold chain, low bioburden, avoidance of repeated freeze–thaw) apply. (Validated shelf-life in final formulations: Not established in the literature.)

Preclinical Evidence

Animal and in vitro studies

Reduced IGFBP binding and receptor potency (in vitro).
Biochemical and structural studies conclude that Arg³ substitution and the N-terminal extension reduce IGFBP affinity while maintaining IGF1R binding; multiple analog screens identified A-/B-domain residues that govern receptor specificity vs insulin receptor cross-reactivity. LR3 preserves IGF1R agonism with lower IGFBP-mediated sequestration, explaining its robust in-vitro mitogenicity. PubMed+1

Cell culture (CHO/HEK and other mammalian lines).
LR3 is frequently used at low-to-mid nanomolar concentrations to sustain signaling and viability in serum-free media, increasing viable cell density and product titers in bioprocess settings. (This body of data is largely vendor/industry literature; nevertheless, the mechanistic rationale—less IGFBP sequestration—is consistent with primary IGFBP biology.) Repligen

Rodent and pig models (endocrine/organ growth).

• Rats: IGF analogs with lowered IGFBP binding stimulated growth in some studies. (Heterogeneous literature; exact LR3 protocols vary.)

• Pigs: In contrast, Long R³ IGF-I infusion increased organ weights but reduced plasma IGF-I/IGF-II/IGFBP-3and inhibited net body-weight gain, illustrating species-specific and axis feedback effects. (Investigational continuous infusion; Conlon 1995.) PubMed

• Finisher pigs: LR3 reduced growth rate and circulating GH/IGFBP-3 and endogenous IGF-I; the effect was partially modulated by co-administration of porcine GH. (Investigational doses; Dunaiski 1998.) PubMed

Representative investigational amounts (preclinical):

• Long R³ IGF-I infusion in pigs (continuous; protocol-specific) — “investigational dose used in study Conlon 1995”. PubMed

• Repeated LR3 dosing in pigs (protocol-specific) — “investigational dose used in study Dunaiski 1998.” PubMed

• Cell culture typically uses 1–100 nM ranges (formulation- and line-specific) — “investigational concentrations in bioprocess literature.” Repligen

Comparative efficacy and safety (preclinical).
LR3 tends to outperform native IGF-1 in serum-containing media (where IGFBPs are present), but in vivo outcomes vary with species, dose form (bolus vs infusion), developmental stage, and GH/IGF feedback. Adverse phenomena in animals may include axis suppression and disproportionate organ growth when exposure is unphysiologically sustained. (Formal GLP tox packages specific to LR3 are not publicly available—Not established.) PubMed

Limitations.

• The most informative in vivo work is species-specific and sometimes decades old.

• LR3-specific receptor binding constants and tissue distribution in vivo are not comprehensively standardized across laboratories.

• IGFBP biology can locally enhance IGF1R signaling (e.g., matrix-bound IGFBP-5) rather than solely inhibit it, thus lower-IGFBP-affinity ligands may not be uniformly “more effective” across tissues. PMC

Human Clinical Evidence

Summary stance. As of September 26, 2025, there are no registered Phase I–III interventional trialsspecifically evaluating IGF-1 LR3 for therapeutic endpoints in humans found in major registries. Human PK/PD, efficacy, and safety for LR3 remain Not established.

IGF-axis clinical context (comparators)

• Mecasermin (rhIGF-1) has been clinically developed (distinct native sequence), providing human precedent for IGF1R agonism under regulated dosing/monitoring. However, results with native IGF-1 do not translate to LR3without dedicated trials because IGFBP interactions, PK, and tissue distribution differ. (Comparative inference only; not a substitute for LR3 data.)

• Cancer trials targeting IGF1R (antibodies, TKIs) and metabolic/endocrine indications inform risk frameworks for IGF1R pathway modulation, but they do not constitute LR3 evidence. BioMed Central

Investigational doses (humans)

• None for LR3 under peer-reviewed, registered interventional studies suitable for citation here. (Not established.)

Safety signals/adverse events

• Human LR3 safety: Unknown/Not established.

• Extrapolations from IGF-axis modulation suggest theoretical risks (see Safety & Toxicology), but direct LR3 safety signals in controlled human studies are lacking.

ClinicalTrials.gov IDs

• No LR3-specific interventional trial IDs located. (If small non-indexed pilot uses exist, they are not part of authoritative registries and are not included.)

Comparative Context

Related peptides and growth-factor ligands

• Native IGF-1 and IGF-2: endogenous ligands; IGF-2 preferentially engages IGF2R (clearance) and IGF1R.

• Insulin: shares structural/functional ancestry with IGF-1; IR/IGF1R hybrid receptors alter ligand pharmacology in tissues. Nature

• Other engineered IGF analogs: point mutants mapping receptor vs IGFBP determinants (e.g., A-/B-domain studies), but LR3 is unique in combining Arg³ with N-terminal extension to minimize IGFBP binding. PubMed

Advantages (research perspective)

• Reduced IGFBP sequestration → higher free ligand in IGFBP-rich environments (e.g., serum-containing media, certain interstitial spaces).

• Preserved IGF1R agonism with robust in vitro trophic signals; widely used to standardize low-serum culture. Repligen

Disadvantages / constraints

• Axis feedback and species-specific responses in vivo; paradoxical growth inhibition observed in pigs despite organ enlargement. PubMed

• Clinical translation gap: no human PK/PD/safety dossier; unknown long-term risks.

• Mechanistic nuance: IGFBPs can potentiate signaling in matrix contexts; reducing their interactions is not uniformly advantageous across tissues. PMC

Research category placement

• Receptor agonist tool compound for IGF1R with engineered IGFBP-evasion, used in cell biology and comparative endocrinology; not a licensed therapeutic.

Research Highlights

• Design rationale validated: Arg³ + N-terminal extension → lower IGFBP affinity with maintained IGF1R binding; NMR suggests both electrostatic and conformational contributions. ScienceDirect

• In vitro potency: LR3 frequently outperforms native IGF-1 in IGFBP-rich media, increasing viable cell density and productivity in bioprocess settings. Repligen

• In vivo complexity: Pig studies show organ hypertrophy but reduced overall growth and suppressed endogenous axis under LR3 exposure—underscoring the complexity of feedback and binding-protein biology. (Investigational infusion/dose regimens.) PubMed+1

• Core signaling conserved: LR3 activates canonical IGF1R→PI3K–AKT/mTOR and RAS–MAPK cascades that regulate growth, survival, and differentiation. PMC+1

Conflicting/uncertain areas

• Human PK/PD of LR3 (half-life, volume of distribution, bioavailability by any route) — Not established.

• Tissue-specific outcomes where IGFBP-matrix interactions facilitate rather than inhibit IGF action — context-dependent/under study. PMC

Potential Research Applications (no clinical claims; research-use framing)

1. Receptor pharmacology & signaling kinetics

   • Time-resolved AKT/ERK profiling under native IGF-1 vs LR3 ± IGFBPs (1–6) and matrix-bound IGFBP-5 to quantify presentation vs sequestration effects. PMC

2. IGFBP biology and extracellular matrix (ECM)

   • Examine how IGFBP-5 ECM binding lowers its affinity for IGF-1 and can enhance receptor access; determine whether LR3’s reduced IGFBP affinity overrides or dampens such facilitation in 3D matrices. PMC

3. Hybrid receptor mapping

   • Compare IGF1R homodimers vs IR/IGF1R hybrids with LR3 vs native IGF-1 to resolve ligand bias and downstream phosphoproteomes in metabolic vs mitogenic contexts. Nature

4. Endocrine feedback modeling (in vivo)

   • Controlled rodent vs pig studies to dissect GH, IGF-I/II, and IGFBP-3/ALS responses under bolus vs infusion, identifying regimes that separate organ-specific trophic from whole-body growth outcomes. PubMed

5. Bioprocess optimization

   • Head-to-head tests of LR3, native IGF-1, and insulin in CHO/HEK platforms under low-protein media, correlating cell-density/viability with secretome IGFBP levels to formalize selection rules for media design. Repligen

Safety & Toxicology

Preclinical toxicity data

Dedicated GLP repeat-dose, genotoxicity, reproductive, and carcinogenicity studies specific to LR3 are not publicly available. General IGF1R agonism stimulates mitogenic and anti-apoptotic signaling; sustained exposure can theoretically favor neoplastic processes in permissive contexts, a concern that underlies oncology programs targeting IGF1R (antagonism). LR3-specific long-term tox in any species at pharmacologic exposure remains Not established. BioMed Central

Known/theoretical molecular risks (inferred from pathway biology)

• Axis dysregulation (GH suppression; altered IGFBP-3/ALS; changes in endogenous IGF-I/II). Observed in pig studies with Long R³ IGF-I. PubMed

• Mitogenic/anti-apoptotic bias via PI3K–AKT–mTOR and MAPK, with tissue-specific oncologic concerns (theoretical without LR3 human data). Nature

• Hypoglycemic signaling via IR/IGF1R hybrids is theoretically possible in some tissues, but LR3-specific receptor bias in vivo is Unknown. Nature

Human safety observations

There is no controlled human safety dataset for LR3. Any extrapolation from native IGF-1 or cell-culture usage should be considered hypothesis-generating only.

Data gaps

• Human PK/PD, dose-exposure–response, immunogenicity (anti-drug antibodies), organ-selective effects, and long-term safety remain Not established.

Limitations & Controversies

• Clinical evidence gap: LR3 lacks registered human trials with published outcomes; translation from in vitro/animal data is therefore limited.

• IGFBP paradoxes: IGFBPs may inhibit or present IGF-1 depending on ECM binding and proteolysis; minimizing IGFBP binding (as with LR3) may not be uniformly beneficial across tissues. PMC

• Species-specific endocrine feedback: The pig data show that organ-level trophic effects do not guarantee net growth, emphasizing axis homeostasis. PubMed

Future Directions

• Quantitative LR3 PK/PD (preclinical → human): Define clearance, volume, bioavailability by route, and IGFBP occupancy (mass-spec/ligandomics) vs native IGF-1. (Currently not available in humans.)

• Tissue-resolved signaling: Phosphoproteomic mapping of LR3 vs native IGF-1 across IGFBP-rich vs IGFBP-poor microenvironments to test whether LR3 preferentially drives mitogenic vs metabolic outputs.

• Endocrine-systems models: Coupled GH–IGF–IGFBP–ALS simulations validated in rodent/pig to predict axis adaptations under pulsatile vs sustained LR3 exposure.

• Safety maturation: GLP-grade repeat-dose and oncology-relevant studies (tissue hyperplasia, preneoplasia markers) plus immunogenicity profiling.

References

1. Clemmons DR. IGF binding proteins and their functions. Endocr Rev. 1993;14(1):55–71. PMID: 7691098. PubMed

2. Allard JB, Duan C. IGF-binding proteins: Why do they exist and why are there so many? Front Endocrinol (Lausanne). 2018;9:117. PMCID: PMC5900387. PMC

3. Werner H, Bruchim I. The IGF1 signaling pathway: from basic concepts to clinical relevance. Cancers (Basel).2023;15(14):3583. PMCID: PMC10573540. PMC

4. Shooter GK, et al. IGF-I A- and B-domain analogues map receptor/ligand determinants. J Mol Endocrinol.1996;17(3):245–258. PMID: 8981230. PubMed

5. Laajoki LG, et al. Solution structure/backbone dynamics of Long-[Arg³] IGF-I; implications for reduced IGFBP binding beyond charge effects. J Biol Chem. 2000;275(41):31488–31495. doi:10.1074/jbc.M003063200. ScienceDirect

6. Conlon MA, et al. Long R³ IGF-I infusion stimulates organ growth but reduces plasma IGF-I/IGF-II/IGFBPs in pigs. J Endocrinol. 1995;146(1):47–57. PMID: 7561636. (Investigational infusion regimen.) PubMed

7. Dunaiski V, et al. Long R³ IGF-I reduces growth and circulating GH/IGFBP-3/IGF-I in finisher pigs; interactions with porcine GH. J Endocrinol. 1998;155(3):559–565. PMID: 9488001. (Investigational dosing.) PubMed

8. Walton PE, et al. ELISA quantitation and pharmacology context: Long-Arg³ IGF-I has much lower IGFBP affinity than native IGFs. J Endocrinol. 1998;156(3):407–417 (methods paper). (PDF). Bioscientifica

9. Repligen. LONG® R³ IGF-I cell-culture supplement (mechanistic/process context). (Accessed 2025-09-26). Repligen

10. Bach LA. IGFBPs overview. Endocr Relat Cancer. 2017;24(6):R131–R150. PMID: 29255001. Europe PMC

11. Soni UK, et al. IGF-1R targeting in cancer; PI3K-AKT-mTOR & RAS-MAPK cascades and localization issues. J Exp Clin Cancer Res. 2023;42:281. BioMed Central

12. He Y, et al. Targeting PI3K/AKT signal transduction for cancer therapy (pathway context). Signal Transduct Target Ther. 2021;6:425. Nature

13. Ferry RJ Jr., et al. Cellular actions of IGFBPs (matrix-bound IGFBP-5 may potentiate IGF-I). Endocr Rev.1999;20(6):761–787. PMCID: PMC4151550. PMC

14. GenScript. Long R³ IGF-I technical description: 83 aa, Arg³ substitution + 13 aa extension. (Accessed 2025-09-26). GenScript

Notes on investigational amounts:
• Long R³ IGF-I infusion in pigs (continuous IV; protocol-specific) — investigational dose used in study Conlon 1995. PubMed
• Repeated LR3 dosing in pigs (protocol-specific) — investigational dose used in study Dunaiski 1998. PubMed
• In vitro bioprocessing typically uses 1–100 nM ranges — investigational concentrations based on vendor/industry media optimization literature. Repligen

⚠️ Disclaimer This peptide is intended strictly for laboratory research use. It is not FDA-approved or authorized for human use, consumption, or therapeutic application.